Without robots, there would be no safe way to clean up the crippled Fukushima Daiichi nuclear power plant.

The problem is that recently, some of the robots being used in the most dangerous and critical part of the cleanup of the 2011 nuclear plant disaster failed. The robots succumbed to massive amounts of radiation or got stuck in rubble.

On March 11, 2011, hydrogen explosions ripped through the plant when a devastating earthquake and tsunami struck Japan. The power plant suffered catastrophic meltdowns in three of its six nuclear reactors.

Six years later, the environment inside the facilities remains too dangerous for people, and even for some robots.

The cleanup and decommissioning of the Fukushima power plant, which is expected to take 30 to 40 years and cost tens of billions of dollars, depends on robots of various sizes and shapes. There are robots that are snake-like, others shaped more like a scorpion, some that are tiny and others that weigh 500 pounds. Machines from around the world have been sent to aid in the cleanup.

"I don't think they can do it without robots," said Taskin Padir, an associate professor at Northeastern University's Robotics and Intelligent Vehicles Research Laboratory. "We'll never be able to send humans into the reactor. The levels of radiation are unheard of. Contamination and radiation are huge. There's no way people are going in."

During the disaster, nuclear fuel rods were displaced, broken and melted. Removal of this radioactive debris is the most critical part of the cleanup and decommissioning operation.

Some of these robots are doing significant work. They’re taking measurements, assessing damage and recording images and video, among other jobs.

Tokyo Electric Power Co.

An image captured by a robot inside the reactor 1 pressure vessel at the Fukushima Daiichi nuclear power plant.

Robots have assisted in other cleanup efforts after nuclear accidents. They were used after the 1979 accident at the Three Mile Island Nuclear Generating Station in Pennsylvania, and the 1986 disaster at the Chernobyl nuclear power plant in Ukraine.

The entire cleanup of the station's basement at Three Mile Island, which had hundreds of thousands of gallons of contaminated water and debris, was handled robotically.

There have been decades of advances in robotics since the machines were called on to clean up after Three Mile Island and Chernobyl. Scientists have developed autonomous humanoid robots, for example, but those aren’t the machines being used at Fukushima.

Workers involved in the cleanup are relying on old-school, remotely operated robots, instead of autonomous machines. These tried-and-true machines move much like the robots used in the late 1970s and 1980s. What’s different are the advanced sensors and cameras on the robots going into Fukushima, along with improved human and robot interaction.

”We are playing it safe,” Padir said. "In this nuclear robotics community, since these apps are really critical, rather than relying on autonomy, we’re relying on human control… We use proven technology. Say, you have a robot running autonomous code and your collision avoidance algorithms break and [the robot] starts crashing into structures inside the reactor. That could cause more damage."

While the nuclear industry needs to play it safe with the robots being sent into Fukushima, it still needs to get the most radiation-resistant and toughest robots available.

The International Research Institute for Nuclear Decommissioning, or IRID, in 2014 reported that a swimming robot and a crawling robot, both developed by Hitachi GE Nuclear Energy, were able to perform checks using cameras and sensors inside five penetration points in one damaged reactor.

Endeavor Robotics, formerly the Defense & Security unit at iRobot Corp., began delivering robotic systems to the Tokyo Electric Power Company, or Tepco, which owns the Fukushima plant, just 19 days after the 2011 accident.

Since then the Massachusetts-based company has delivered 20 robots, including its 65-pound PackBot system, as well as its Kobra robot. Both machines are equipped with cameras and manipulative arms.

Endeavor Robotics

An Endeavor PackBot robot makes its way downstairs.

The Endeavor robots mainly have been used inside reactor buildings to open doors, perform inspections and check for leaks. The Kobra, which can have a saw attached to one of its two arms, also has been used to move debris.

However, not all of the robots working at Fukushima have been as successful.

The problem here is that many of the robots that have worked well at Fukushima were bigger than earlier robots and were used in and around the buildings. Three robots that recently failed, however, had been sent into the bowels of the reactors.

This is the most critical work — getting into the tight, highly radioactive areas where the fuel needs to be assessed and cleaned up — and the robots needed for that are failing.

”Now that the buildings themselves are more under control, they need to go into the reactors and assess and then remove the fuel,” Padir said. "They need to collect all of that radioactive fuel. Since this is a full shutdown, they need to remove all the fuel in there. They went in and progressed a little bit but were they successfully able to complete the entire mission? No."

Tepco reported last month that a Scorpion robot, developed by Toshiba and the IRID, was on a mission to investigate temperatures and radiation levels in a damaged reactor when it got stuck. The connection cable to the robot was cut and it was abandoned.

Earlier this month, Tepco reported that one robot got stuck in a grating ditch while on its way to performing an inspection.

A lesson learned from that incident, the company said that a laser will be installed on future robots to improve their spatial ability, as well as to enable the machines to check for obstacles and openings while moving. Another robot was lost because radiation caused its camera screen to fail, Tepco said.

A snake-like robot built by Hitachi-GE was lost in 2015 when it was sent in to inspect a nuclear containment vessel, according to a report from the International Business Times. It's unclear why the robot stopped responding but it, too, was abandoned.

These failures are concerning because they are happening at the most critical part of the cleanup.

"Those robots were going into the most damaged reactors,” Padir said. "From a broader perspective, those robots are the most critical. They’re trying to go to an environment where no other robot or no other person could go. There is no way to test your systems in advance. You just have to go in and see what happens. You won’t know how it will perform or what will happen until you send the robot in."

Scientists have already been working on robots that can aid in disaster situations. The 2015 DARPA Robotics Challenge demonstrated two-legged humanoid robots that could climb stairs, walk over piles of rubble, use drills, climb ladders and drive cars, among other tasks. But more research needs to be done before these robots will be ready to aid in such disasters.

Jacopo Buongiorno, a professor of nuclear science and engineering at MIT, said scientists have learned a great deal from the robotics work being done at Fukushima.

MIT researchers are talking with Tepco about a project to develop advanced robots that would overcome the issues their predecessors have had at the power plant site.

Ideas range from robots that use legs instead of tracks, robots that combine walking with swimming, and robots with higher levels of radiation protection.

Padir noted that that scientists are also learning what components on the robots are durable to radiation.

"What will die first? The computer? The camera? How much shielding do you need?” he asked. "We claim that we know how to shield things, but the radiation levels are so high that no one knows, until we put a robot in that situation, how it will do.”

One idea coming out of this work is the possibility of building modular robots, Padir said. If a robot has a six-piece body, for instance, and one piece gets stuck in the rubble, the stuck component could detach itself while the rest of the robot moves ahead with the mission.

Scientists have also learned of a greater need for operator training. The environment that the robots enter at Fukushima is unknown. It could be a tight space, full of rubble and the potential to get stuck or damaged at every turn. This is a major challenge for the robot’s operator, who is working under heavy stress to direct the robot, guided only by what the machine’s sensors and cameras see.

"You’ve got to train these guys as if you’re training F16 pilots because they need to be ready for those unexpected circumstances,” said Padir, who is working on a robotics project with the U.S. Department of Energy.

"This is all generating a lot of new knowledge,” he added. "Every failure is something learned for the robotics community. Everything we learn is going to make the next robot better and greater to take on these challenging tasks."